It is known that steel sheets that are to undergo a hot forming step and a quench hardening must first be heated—at least in some areas—to an austenitizing temperature in order to enable a transformation of the austenite into martensite and thus the production of a hardened structure.
To this end, the structure must be heated, in particular to a temperature above the so-called AC3 point of the steel material.
WO 2013/000001 A1 has disclosed a method for heating a molded part for a subsequent press hardening and has disclosed a continuous furnace in which a molded part, which has been preheated to a predetermined temperature, is heated to a higher temperature in some regions.
The text therein states that hardening structures, which at austenitic steels can yield tensile strengths of greater than 1,500 MPa with an elongation in the vicinity of 6%, are often only required in subregions of the workpiece, while in other regions, higher elongations of for example 15% to 17% are required. Consequently, according to this cited document, a method for heating a molded part to different temperatures should be embodied so that despite a continuous passage through a heating unit, the molded parts receive a heat treatment—which is required for the subsequent press hardening—with an improved temperature control. For this purpose, this document proposes that as it is being fed through a field of heating elements, with the aid of heating elements that are arranged in longitudinal rows and transverse rows relative to the feed direction and can be activated at least in groups with different heating power, the molded part, is heated differently so that parts of the sheet metal reach an austenitizing temperature, whereas other parts of the sheet metal do not reach this temperature. According to this measure, heating elements with different heating capacities can be activated; with the possibility of activating heating elements independently of one another at least in groups, both along longitudinal rows and along transverse rows, it is possible while the components are being conveyed to influence the temperature of the molded parts in a longitudinal strip extending in the feed direction in order to be able not only to achieve but also to maintain preset temperature levels in the region of these longitudinal strips. In addition, cooling devices in the feed direction can also be activated along desired longitudinal strips in order to cool the components in strips.
DE 10 2012 001 742 A1 has disclosed a device for heating sheet metal workpieces for a subsequent hot forming and in particular press hardening. This device should either not have the disadvantages associated with roller hearth furnaces or should only have them to a reduced degree and should have the capacity to be flexibly operated with different heating requirements. To this end, the device should have a plurality of heating stations; a first heating station should be provided, with an inductive or inductively operated heating unit in which the sheet metal workpieces can be quickly heated to a predetermined temperature without dwelling awhile, a second heating station, in which the sheet metal workpieces dwell awhile and are thus heated or cooled to a predetermined temperature or are maintained at a predetermined temperature, and a third heating station in which the sheet metal workpieces dwell awhile and in the process, can be heated or cooled to a predetermined temperature or maintained at a predetermined temperature. Because of the inductive heating principle, the first heating station should enable a rapid and targeted heating of the sheet metal workpieces; the first heating station is preferably embodied as a feed-through station and the sheet metal pieces to be heated are conveyed through the first station individually or possibly also several at a time and without an appreciable dwell time; the heating can take place by means of longitudinal field- or transverse field induction. The second and third heating stations can be operated differently, depending on the heating requirements; the second or third heating station has at least one electrically operated heating unit and at least one heating unit operated by means of fuel combustion. The two heating stations in this case should be embodied as non-feed-through stations, whereas the third heating station is preferably embodied as a feed-through station. In order for the sheet metal workpieces, which are to be heated, to be conveyed or guided through, a main path is provided in which the third heating station is situated, and at least one secondary path is provided, in which the first and second heating stations are situated; the main path and the secondary path have a crossing region, with at least one shunt for transferring the sheet metal workpieces from one path to the other. Preferably, the second heating station is embodied as a non-feed-through station, in particular, the second heating station is embodied as a stacking furnace or has a stacking furnace.
DE 10 2009 019 496 A1 has disclosed a device, which includes three successive heating stations, which the sheet metal workpieces to be heated pass through in sequential fashion. The first heating station in this case has an inductive heater with which the sheet metal pieces to be heated can be quickly heated to a high temperature of up to several 100° C. Then the sheet metal workpieces that have been heated in this way are conveyed to the subsequent heating stations.
DE 10 2009 051 157 B4 has disclosed a method for heating a component for a hot forming, in which the component is heated to a desired temperature in a furnace; the furnace is embodied in the form of a chamber furnace and the internal temperature of the furnace is above the desired temperature of the component at every moment of the heating; when the desired temperature is reached, the component is removed from the furnace without having assumed the excess temperature of the furnace. In this case, the intent is for the method for heating a component for a subsequent hot forming to be improved such that the long heating times in the furnace are reduced and the footprint of the furnace system is significantly reduced.
DE 10 2010 017 905 A1 has disclosed a method for hot sheet forming in which the hot sheet forming is carried, out by heating, a sheet in a first step with inductive heating by means of a first induction heating device to a temperature less than or equal to the Curie temperature and in a second step, the heating to temperatures >800° C. is carried out by means of conventional heating in a furnace or by means of inductive heating in a second induction heating, device that is different from the first. With the method, the heating in the first step can take place, in two sub-steps; in a first sub-step, the sheet is heated to a first temperature and in a second sub-step, the sheet can be kept at a temperature more than 70 K lower than the first temperature. This should enable a homogeneous melting of an AlSi protective coating. In the first step in this case, the protective coating on the sheet can be completely melted; the temperature in this case is close to the Curie temperature, i.e. from 710° C. to 770° C. In the second sub-step, a diffusion process can occur; for AlSi, this lies between 600° C. and 650° C.
DE 10 2009 019 573 A1 has disclosed a furnace and a method for heating at least one workpiece; a workpiece is transported by means of a workpiece support coupled to a transport device, from an intake region to an outlet region of the furnace. The furnace includes two heatable chamber regions, which are situated on top of or also next to each other. The intake and outlet regions of the furnace are likewise situated on top of or next to each other. The workpiece to be heated is first transported on a workpiece support by the transport device through the first chamber region and then through the second; in this case, the transport direction of the workpiece support in the first chamber region is opposite from the transport direction in the second chamber region.
EP 1 830 147 B1 has disclosed a multi-chamber continuous furnace with a protective gas operation and method for oxide-free heating of galvanized workpieces; this is intended to ensure a scale-free heating of galvanized workpieces in a continuous furnace. The furnace in this case is divided into a plurality of chamber regions in each of which protective gas mixtures, preferably of different compositions, are supplied via infeed points, the composition of the protective gas being adapted to the temperature of the workpieces in the respective region of the furnace. The protective gas mixture with the lowest oxygen percentage is supplied in the last chamber region in this case. Between the individual chamber regions, the continuous furnace has corresponding protective gas guidance systems, preferably in the form of dividing walls with openings through which the overall flow of the protective gas can be guided so that a convection roll through the entire continuous furnace is prevented and the speed of the protective gas flow through the continuous furnace is higher than the back diffusion speed. The protective gas mixture is produced through partial combustion of a hydrocarbon/air mixture in a noble metal catalytic converter.
DE 10 2012 104 537 A1 has disclosed a furnace system and a method for operating it in which light alloy components are fed through the furnace system and, inside the furnace system, are heated and possibly cooled with a flow of air or gas.
DE 10 2010 010 156 A1 has disclosed a method for manufacturing a molded part with at least two structure regions of different ductility, this enables the treatment and forming of corresponding semi-finished products or blanks to take place at the cyclical rhythm of the press hardening tool, without influencing the throughput speed through the continuous furnace and in this case, after passing through the continuous furnace, the semi-finished product is inserted with the second subregion into a chamber of a buffer, which keeps the second subregion at the austenitizing temperature, while the first subregion protrudes out from the chamber of the buffer and this protruding region is air-cooled to the temperature at which the ferritic structure is formed.
It should be generally noted that press hardening is a very challenging technique in which it is important to insert the ready-heated blanks or semi-finished products into the tool at the right time, to press them, and to thus cool them and harden them at the same time.
In addition, it is important for the components to in filet be austenitized in the regions that are to be hardened or for them to be completely austenitized, but it is not desirable for the components to be left in a furnace for longer than is necessary.
While many furnaces are embodied as continuous furnaces, i.e. ones that can be loaded and unloaded continuously, the press is operated cyclically so it is very challenging to coordinate the loading and unloading to and from the furnace on the one hand and the furnace dwell time on the other with the cyclical operation of the press.
By and large, it is desirable that with a sheet thickness of 1 mm to 2 mm, at most 0.8 mm to 3 mm, as well as a cycle time of 10 seconds and a target temperature of the blank of 900° C. with a dwell time of approx. 5 seconds, a homogeneity of +/−15° C. throughout the blank is achieved.
The heating methods mentioned in the prior art are in particular carried out by means of conduction, induction, or thermal radiation, in particular by means of open gas flames. It has turned out that none of these heating forms actually meets the requirements completely throughout the entire process.
Known furnaces for radiative heating include, for example, chain conveyor furnaces, roller hearth furnaces, lifting step conveyor furnaces, and multi-chamber furnaces.
In the methods and devices known front the prior art for radiative heating of components, as a rule, relatively long furnace times are required in order to heat the components to the desired temperature.
In addition, with heating types of this kind, either protective gas is used or oxygen is allowed to contact the component surface unhindered.
With galvanized components, the long furnace times of the kind that are necessary, for example, in radiation furnaces, result in a variety of disadvantages. These disadvantages lie in the fact that the gamma phase breaks up relatively quickly, the Fe content increases, and the electrochemical potential increases. In addition, the processing windows only last a limited time. Because of the short cycle time that is required in order to achieve economical operation, relatively long furnaces are frequently used, which are loaded with a plurality of parts one after the other. If there are problems or interruptions in the press or in the furnace, then this increases the amount or resulting rejects because the upper limit of the processing window is exceeded for all of the numerous parts in the furnace.
If an intense oxide formation occurs with galvanized components, these parts must then be cleaned.
But if an oxygen-free atmosphere is used with galvanized components, oxidizing loss usually occurs since no oxidation layer (or only a thin one) is able to form.
EP 2 014 777 B1 has disclosed a method and a device for thermally treating sheet metal. In this connection, a metal component is heated in a first method step and in a subsequent method step, is brought into contact with at least one contact plate. In this connection, the contact plate has a lower temperature as compared to the metal body. Due to the lower temperature of the contact plates, the metal body is cooled after the heating. In this case, it should be possible for this cooling by means of the contact plates to be favorably controlled in terms of the temperature control.
DE 10 2011 053 672 A1 has disclosed a method and an apparatus for heating a metal blank. In this case, a metal blank must be heated in a heating device; the heating device has at least one lower contact element and at least one upper contact element so that between the contact elements, which are adapted to the contour of the metal blank, the blank is heated to a temperature of 200° C. to 450° C. by supplying thermal energy.
DE 10 2011 102 167 A1 has disclosed a method for manufacturing a molded part with at least two structure regions of different ductility and has disclosed a heating unit. According to this method, a blank in a heating unit should contact at least one heating plate having at least two heating segments in such a way that the first heating segment, which is set to a temperature A, heats a first region of the blank to a temperature A and the second heating segment, which is set to a temperature B, heats the second region of the blank to a temperature B, with the heating of the first region and second region of the blank each occurring by means of thermal conduction.
EP 2 182 081 A1 has disclosed a method and a device for thermally treating a coated steel body. In this case, before a hot forming process, a sheet steel body is brought into contact with at least one first contact plate having at least one first surface section of the sheet steel body and at least one second contact plate having at least one second surface section of the sheet steel body. During the contacting, one contact plate should have a higher temperature than the sheet steel body.
EP 2 237 639 A1 describes a device and a method for heating, a body. In this case, a contact plate, which has at least two heating elements, is provided in order to contact the body that is to be heated.
EP 2 395 116 A2 has disclosed a device for heating steel plates. By means of this device, it should be possible to use a contact plate to heat different regions of the steel plate to different temperatures.
JP 2007-245196 A and JP 2009-095869 A have disclosed devices for contact heating.
WO 2010/048950 A1 describes a method for thermally treating a coated sheet steel body. According to this method, at least one contact plate for heating the sheet steel body has a higher temperature greater, than Ac3 during the contacting, in particular a temperature between 20° C. and 250° C. from Ac3, so that an austenitization of the sheet steel body takes place.
WO 2012/045647 A1 describes a method and a furnace for treating workplaces. In this case, a workpiece should be heated by at least two heating elements in the furnace. In particular, the heating units have heatable pressure pistons.
The object of the invention is to create a method for heating steel sheets for hot forming and hardening purposes, which allows these steel sheets to be reliably and reproducibly brought to the necessary temperature.
Another object of the invention is to create a device for carrying out the method.